When steel is refined in an electric furnace or a converter, iron scrap to be fed into the furnace, which is a source of iron, is previously heated. In this preheating process, a large quantity of thermal energy is required. Therefore, from the viewpoint of operating cost, the preheating of iron scrap does not show much merit to reduce the cost and, further, the equipment cost is increased to provide the preheating apparatus. Recently, from the viewpoint of recycling the resource, the recycling of iron scrap has become important. In order to conduct recycling iron scrap effectively, in order to use iron scrap effectively from the viewpoint of putting it into practical use as an iron source and in order to save energy when iron scrap is used, it has been desired to develop a preheating technique for preheating iron scrap by which the total quantity of energy required for refining can be reduced as much as possible and the production cost can be reduced.
The prior art in this technical field of melting iron scrap was disclosed in "Strategy for Electric Furnace for Common Steel", which was published in November of 1994, in "Electric Furnace" issued by The Iron and Steel Institute of Japan and presented in the Twenty-seventh and Twenty-eighth Shiraishi Memorial Lectures. According to the above prior art, when iron scrap is melted, it requires a large electric power consumption, so that the operation cost is raised. According to the description of the above prior art, a cause of increase in the operation cost is a deterioration in the thermal efficiency after the step of flat bath (at the time of melting down). Concerning the method of melting iron scrap in a converter type furnace, there is a report on page 520 in vol. 78 (1992) in "Iron and Steel" (issued by the Iron and Steel Institute of Japan). According to the report, the equipment cost is increased to process a large quantity of exhaust gas and, since iron scrap is refined into molten iron, it is necessary to provide a converter in the after-process. According to the U.S. Pat. No. 4,852,858, there is disclosed a method of preheating material by the exhaust gas discharged from a refining furnace when a shaft-shaped material accommodating container is arranged in an upper portion of the metal refining furnace. However, according to the above technique, it is difficult to control the preheating condition constantly. Therefore, the refining condition tends to fluctuate greatly. As a result, it is difficult to stabilize the product quality. Japanese Examined Patent Publication (Kokoku) No. 6-46145 discloses another shaft-type preheating furnace. According to this shaft-type preheating furnace, although the thermal efficiency is high, a lower portion of the furnace is exposed to gas of high temperature. Accordingly, pieces of iron scrap are fused to each other. Therefore, it is necessary to cut off the pieces of iron scrap, which have been fused to each other, by means of oxygen cutting. Further, the following are disclosed. When water-cooled grates are provided, the charging ratio is lowered in order to ensure a space in which the water-cooled grates are operated. Therefore, the thermal efficiency is deteriorated.
Concerning the rotary kiln used for preheating iron scrap, Japanese Unexamined Patent Publication (Kokai) No. 6-228662 discloses that it is difficult for pieces of iron scrap to be fused to each other since the rotary kiln is always rotated so as to move the pieces of iron scrap in it, however, the charging ratio of iron scrap is low, so that the thermal efficiency is deteriorated. Usually, iron scrap not containing organic material is heated in the above rotary kiln for preheating iron scrap. Therefore, it is necessary to conduct a previous treatment of iron scrap (classification). As a result, the production cost is increased.
Since the above rotary kiln is always rotated so that iron scrap can be oscillated in it, it is possible to introduce gas at a relatively high temperature into the rotary kiln while avoiding the fusion of pieces of iron scrap. With respect to exhaust gas containing combustible components such as CO and H.sub.2, it is possible to change the latent heat into the sensible heat by introducing air or oxygen so that the combustible components can be burned. However, since the charging ratio of iron scrap in the furnace is low, the heat exchanging effectiveness is 30 to 40% at most, which is low. This low heat exchanging effectiveness is the biggest problem. Further, when large and rough pieces of iron scrap and heavy pieces of iron scrap are charged into the rotary kiln type preheating furnace, there is a possibility that the inside refractories of the preheating furnace are damaged.
On the other hand, when the shaft furnace type preheating furnace is used, it is possible to accomplish a high heat exchanging effectiveness because iron scrap is charged into the preheating furnace and heat exchange is conducted between iron scrap and exhaust gas. However, in accordance with a decrease in gas temperature, the heat exchanging effectiveness is deteriorated. Accordingly, the heat exchanging effectiveness is actually about 70% at most. When small pieces of iron scrap are exposed to gas of high temperature in the furnace bottom, they are heated to temperatures not lower than 800.degree. C., so that the pieces of iron scrap are fused to each other or spaces formed between the pieces of iron scrap are filled with dust. As a result, the pressure loss is increased. In an example, in order to avoid the fusion of pieces of iron scrap, water-cooled grates are provided, and the pieces of iron scrap are put on the grates and preheated. However, in the above example, in order to ensure the operating space in which the grates are operated, the fusing ratio of iron scrap in the furnace is decreased, and the heat exchanging effectiveness is lowered. In order to solve the above problems, Japanese Unexamined Patent Publication (Kokai) No. 7-180975 discloses an example in which piece of iron scrap, the sizes of which are so small that they may be fused to each other when they are preheated, are charged into an arc furnace from an exclusive charging port without being preheated so that the occurrence of fusion can be prevented. However, this example causes a problem that a portion of iron scrap is charged into the furnace without being preheated.